This invention relates to lithography, and more particularly to a sulfanilic acid-containing concentrate, which when mixed with water, provides a fountain solution for lithography.
Conventional lithography relies on the mutual incompatibility of two fluids, ink and water, in its operation to produce printed matter containing sharp images on clean backgrounds. In theory, a presensitized lithographic printing plate such as is commonly used in the industry, for example as disclosed and claimed in U.S. Pat. Nos. 3,396,019 and 3,929,591, when exposed to actinic radiation through an original transparency and developed with suitable solvents, results in an image area which is oleophilic, and therefore suitable to accept greasy, lithographic printing ink and background areas which are hydrophilic in character and capable of accepting water, or more generally, an aqueous, acidic fountain solution to thereby resist the presence of ink in such background areas of the printing plate.
Rather narrow operating conditions typically prevail on a lithographic press between the relative amounts of ink and fountain solution utilized so that the printed sheet shows no evidence of ink in the background areas, but yet densely reproduces the colors of the original subject matter in image areas. For example, excess fountain solution, or, an improperly prepared solution, may cause blinding of the oleophilic regions of the plate, thereby resulting in a washed out or weak-looking print. Conversely, insufficient fountain solution covering the background areas of the plate may cause "catch-up", toning, or scumming of the background areas.
Similarly, the pH of the fountain solution must be relatively specific for trouble-free operation. If the pH number is too low, there can occur, among other conditions, ink emulsification, tinting, scumming, snow flaking, rapid plate wear, and slow ink drying. If the pH number is high, it is possible to have sensitivity in the background areas, emulsification, half tone plugging, and dirty dampeners. In most instances, a pH range of from about 3.5 to about 5.0 has been determined to be most commonly used, with a mid-range value of about 4.0 being the general target.
In addition to problems relating to the lithographic printing plate itself, there can exist conditions related to the press function whereby a phenomena called "ink stripping" may occur. This deficiency may be caused by the fountain solution ingredients forming a film on the metal rollers of the press and thereby preventing the ink from being transferred from one roll to the adjacent roll in the ink train. For example, when there is too much acid in the fountain solution, this ink stripping may occur, or if gum arabic is used as the desensitizer, acidic gum may coat the roller surfaces making them hydrophilic in character, as opposed to oleophilic.
These and other problems have been recognized by experts in the lithographic field and many procedures have been attempted to minimize adverse effects. For example when the lithographic support or base consists of an aluminum substrate with an anodized coating thereon, which typically represents the hydrophilic background region after image development, a subbing layer may be provided which contains a hydrophilic cellulosic compound and a metal salt over which can be coated a layer of actinic radiation-sensitive polymer composition, as is taught in British Pat. No. 1,442,760.
Furthermore, British Pat. No. 1,414,575 discloses subbing compositions exhibiting extremely high ink/water differential between the image and non-image areas so as to assist in prevention of scumming on "roll-up" or subsequent use of the plate.
In addition to treatment of the printing plate itself to prevent scumming, blinding, etc., considerable effort has been expended to prepare desensitizer formulations which would maintain the ink/water differential on a lithographic plate surface during press operation. U.S. Pat. No. 3,738,850 discloses the addition of dialdehydes to desensitizer formulations so that the plate has less tendency to scum, particularly after the developed plate has been stored for a period of time.
The acidic pH-adjusting component of conventional fountain solution is phosphoric acid. Phosphoric acid, aside from toxicity of waste materials, has been thought to induce detrimental problems in the printing process, typically through the lack of control of the concentration utilized by the press operator. If this acid is used in excess in the fountain solution, the cobalt driers conventionally contained in lithographic ink can be adversely affected, such that the drying of the ink on the printed sheet becomes retarded. Such insufficient drying results in ink "setting off" to the next adjacent sheet in the stacker.
In the case of anodized aluminum substrates, the resin image areas thereon are subject to undercutting, flaking and subsequent removal in the printing operation when strong concentrations of phosphoric acid are utilized in the fountain solution. Furthermore, it has been reported that phosphoric acid may operate in a deleterious fashion on the anodized aluminum surface itself to soften the structure and in fact cause the plate to become sensitized, i.e., the background areas of the plate pick up ink in isolated, discrete spots, which results in ink toning on the background areas of the printed sheet.
It has now been found that by utilizing sulfanilic acid as the acidic component of the fountain solution, excellent results can be obtained. Furthermore, by utilizing sulfanilic acid, a reduced amount of the hydrophilic colloid or desensitizer typically contained in fountain solutions, e.g., gum arabic, can be utilized, which affords minimizing of plate blinding on the press. Furthermore, sulfanilic acid is not toxic nor is it a pollutant which provides for better waste control of the dampening solution.